Why is my Ford Focus engine so loud?
A failing or dirty sensor can send incorrect data, resulting in too much or too little fuel going to the engine. This can result in a rough running engine that is louder than normal.
Bad or Dirty Spark Plugs: Bad spark plugs can cause the vehicle to misfire which will make it run louder.
An engine that is suddenly running louder than normal is usually crying for help. In the majority of cases an underlying problem is causing the engine to rumble. Problems can range from something simple like dirty spark plugs to the more serious failing muffler or catalytic converter.
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最近对15年车龄，16.5万英里(27万公里）的老车（3.5 V6 2GR-FE）做“大保养”，换下原装火花塞，看它的状况好像问题不大（可能一是DENSO原装正品质量；二是长期好机油勤换+佛系驾驶）：
长期好机油勤换的结果，发动机内部貌似无油腻，很干净：
